The Science Behind Your DUI Charge Is Not as Solid as the Prosecution Wants You to Believe

Every DUI prosecution in Arizona depends on evidence. And in most cases, the centerpiece of that evidence is a number – your BAC result from a blood or breath test. The prosecution will present that number as an objective, scientific fact. They will tell the jury that the test was properly administered, the equipment was properly calibrated, and the result is reliable.

But BAC testing is not as simple or as reliable as the prosecution suggests. Blood tests can be contaminated. Breath tests can be inaccurate. Equipment can malfunction. Procedures can be violated. And the legal framework governing when and how these tests are administered creates constitutional issues that can invalidate the results entirely.

Brad Rideout is a former Arizona District Attorney who relied on blood and breath test evidence to prosecute DUI cases. He knows what the prosecution needs from these tests, what assumptions they make, and where the science and the procedures break down. Now he uses that insider knowledge to challenge the evidence that the state is using against you.

Arizona’s Implied Consent Law – A.R.S. § 28-1321

Arizona operates under an “implied consent” framework. Under A.R.S. § 28-1321, any person who operates a motor vehicle in Arizona is deemed to have given consent to a test of their blood, breath, urine, or other bodily substance for the purpose of determining alcohol concentration or drug content if the person is arrested for DUI.

What Implied Consent Means in Practice

When you are arrested for DUI, the officer will inform you that Arizona law requires you to submit to a chemical test. The officer selects the type of test – you do not get to choose. In most cases, the officer will request a breath test at the station or a blood draw at a medical facility or mobile blood draw unit.

What Happens If You Refuse

If you refuse the chemical test, the consequences are immediate and severe:

12-month license suspension for a first refusal
2-year license suspension for a second refusal within 84 months
The refusal can be used as evidence against you at trial (the prosecution will argue consciousness of guilt)
The officer can obtain a search warrant for a forced blood draw despite your refusal

The refusal does not prevent the state from obtaining your BAC. Since the U.S. Supreme Court’s decision in Missouri v. McNeely (2013) and subsequent Arizona case law, officers routinely obtain telephonic warrants for blood draws within minutes of a refusal. The warrant process has become so streamlined that refusing rarely prevents a blood draw from occurring.

However, there are situations where refusal may be strategically appropriate. The delay between the refusal and the warrant-authorized blood draw may result in a lower BAC at the time of testing due to the body’s natural elimination of alcohol. Whether refusal is advisable depends on the specific circumstances, and this is a decision that should be made with legal counsel when possible.

Breath Testing in Arizona

The Intoxilyzer 8000

Arizona law enforcement primarily uses the Intoxilyzer 8000, manufactured by CMI Inc., for evidentiary breath testing. This is the instrument at the police station or DUI processing center – not the handheld preliminary breath tester (PBT) used at the roadside.

The Intoxilyzer 8000 uses infrared spectroscopy to measure the concentration of alcohol in a breath sample. The device passes infrared light through the breath sample and measures how much light is absorbed at specific wavelengths associated with ethyl alcohol. The amount of absorption is converted to a BAC reading.

How Breath Testing Can Produce Inaccurate Results

The Intoxilyzer 8000 does not directly measure blood alcohol concentration. It measures breath alcohol concentration and uses a mathematical conversion factor (the blood-to-breath ratio) to estimate BAC. This conversion assumes a standard blood-to-breath ratio of 2100:1, meaning that 2,100 milliliters of breath contain the same amount of alcohol as 1 milliliter of blood.

The problem is that this ratio varies significantly from person to person. Studies have documented individual blood-to-breath ratios ranging from 1,300:1 to 3,100:1. The standard 2100:1 ratio is an average, not a constant. If your individual ratio is lower than 2100:1 (which it is for a significant percentage of the population), the Intoxilyzer 8000 will overestimate your BAC.

Additional sources of breath test inaccuracy include:

Mouth alcohol. If alcohol is present in your mouth at the time of testing – from a recent burp, acid reflux, dental work trapping alcohol, or residual alcohol from a drink consumed shortly before the test – the device will read the concentrated mouth alcohol rather than the deep lung air it is designed to measure. Arizona protocol requires a 15-minute observation period before testing to ensure mouth alcohol dissipates, but officers do not always comply with this requirement.

Temperature variations. Breath testing assumes a standard body temperature of 34°C for exhaled breath. If your body temperature is elevated due to fever, physical exertion, or being in a hot police car, the alcohol concentration in your breath will be higher than the alcohol concentration in your blood. A body temperature increase of 1°C can increase the breath test reading by approximately 6-8%.

Breathing patterns. Hyperventilation before a test can lower the reading, while holding your breath can increase it. The depth and duration of the breath sample also affect results. Officers sometimes instruct subjects to blow harder or longer, which can produce artificially high readings.

Radio frequency interference (RFI). Electronic devices, police radios, and other sources of electromagnetic radiation can interfere with the Intoxilyzer 8000’s readings. The device has an RFI detector, but its effectiveness is debated. Testing conducted near police radio equipment or in rooms with significant electronic activity may produce unreliable results.

Calibration errors. The Intoxilyzer 8000 must be regularly calibrated using reference solutions of known alcohol concentration. If the calibration is off, every test conducted on that instrument until the next calibration will be affected. Defense attorneys can subpoena calibration records, maintenance logs, and quality assurance data to verify that the instrument was functioning properly at the time of the test.

Interfering substances. The Intoxilyzer 8000 is designed to measure ethyl alcohol (the alcohol in beverages). However, other compounds can absorb infrared light at similar wavelengths, potentially producing false positive readings or artificially elevated results. Acetone (produced by diabetics and people on low-carbohydrate diets), certain industrial solvents, and some medications can trigger interference.

The Two-Test Protocol

Arizona requires that two breath samples be collected within a specified time period. The two results must agree within a defined tolerance (typically 0.020). If the results do not agree, the test is considered invalid and must be repeated.

This two-test requirement is a built-in quality control measure, but it is not foolproof. Two inaccurate readings can still agree with each other if the source of error affects both samples equally (such as an uncalibrated instrument or elevated body temperature).

Blood Testing in Arizona

When Blood Tests Are Used

Blood testing is generally considered more accurate than breath testing and is preferred in several situations:

When the suspect is involved in an accident and transported to a hospital
When the suspect is suspected of drug impairment (breath testing detects only alcohol)
When the suspect refuses breath testing and a warrant is obtained for a blood draw
When the Intoxilyzer 8000 is unavailable or malfunctioning

The Blood Draw Process

Forensic blood draws must be performed by qualified personnel – typically a phlebotomist, nurse, or EMT trained in blood collection. The blood is drawn into vacuum-sealed tubes (gray-top Vacutainer tubes) that contain preservatives (sodium fluoride) to prevent fermentation and anticoagulants (potassium oxalate) to prevent clotting.

The tubes are sealed, labeled, and maintained in a documented chain of custody from the time of collection to the time of analysis at the crime laboratory.

How Blood Test Results Can Be Challenged

Chain of custody failures. The blood sample must be tracked from the moment it leaves your body to the moment it is analyzed. Every person who handles the sample, every transfer between facilities, and every storage condition must be documented. Gaps in the chain of custody raise questions about whether the sample was properly handled, stored at the correct temperature, and protected from contamination or substitution.

Fermentation. If blood samples are not properly preserved or stored, the blood can ferment, producing alcohol that was not present at the time of collection. Fermentation can increase the measured BAC by 0.01 to 0.04 or more. Factors that promote fermentation include:

Insufficient sodium fluoride preservative in the collection tube
Elevated storage temperatures
Delayed analysis (the longer the sample sits, the more opportunity for fermentation)
Contamination with bacteria or yeast during collection

Improper collection procedures. The phlebotomist must use a non-alcohol swab to clean the draw site. Using an alcohol-based swab can contaminate the sample and produce an artificially elevated result. The collection tubes must not be expired. The tubes must be properly mixed by inversion after collection to distribute the preservative and anticoagulant.

Laboratory errors. Crime laboratories analyze blood samples using gas chromatography, a highly accurate method when performed correctly. However, laboratories can make errors in sample preparation, instrument calibration, data interpretation, and reporting. Quality assurance records, analyst training documentation, proficiency testing results, and laboratory accreditation status can all be relevant to challenging blood test results.

Retrograde extrapolation issues. There is almost always a time delay between the DUI stop and the blood draw. During that delay, the body continues to absorb and eliminate alcohol. The prosecution may attempt to use retrograde extrapolation to estimate your BAC at the time of driving based on your BAC at the time of the blood draw. This calculation relies on assumptions about your absorption and elimination rates that may not be accurate for you individually. If you were still in the absorptive phase at the time of driving, your BAC at the time of the blood draw may actually be higher than your BAC at the time of driving.

Missouri v. McNeely and Warrant Requirements

The Constitutional Framework

In Missouri v. McNeely (2013), the U.S. Supreme Court held that the natural dissipation of alcohol in the bloodstream does not constitute a per se exigency that justifies a warrantless blood draw in every DUI case. The Fourth Amendment generally requires law enforcement to obtain a warrant before conducting a blood draw.

This decision changed DUI enforcement nationwide. Before McNeely, many jurisdictions allowed warrantless blood draws based on the theory that the evidence (alcohol in the blood) was being destroyed in real time as the body metabolized it. The Supreme Court rejected that argument, holding that the availability of telephonic warrants and electronic warrant applications provides a practical mechanism for officers to obtain warrants quickly without significant delay.

How This Applies in Arizona

Arizona has implemented streamlined warrant procedures that allow officers to obtain blood draw warrants within minutes via telephone or electronic submission. Many jurisdictions have on-call judges available 24/7 specifically for DUI blood draw warrant applications.

Despite the efficiency of the warrant process, there are still situations where officers draw blood without a warrant. When this occurs, the defense can challenge the admissibility of the blood test results under the Fourth Amendment. The prosecution must demonstrate that an exception to the warrant requirement applies – typically exigent circumstances beyond the normal dissipation of alcohol.

Warrant Validity Challenges

Even when a warrant is obtained, the warrant itself can be challenged:

Insufficient probable cause – The warrant affidavit must establish probable cause to believe the suspect was driving under the influence. If the affidavit contains errors, omissions, or conclusory statements that do not support probable cause, the warrant may be invalidated.
Staleness – The warrant must be executed within a reasonable time. Excessive delay between warrant issuance and blood draw can render the results unreliable.
Scope – The warrant must specifically authorize a blood draw. A warrant that authorizes only breath testing does not support a blood draw.
Magistrate neutrality – The judge who issues the warrant must act as a neutral and detached magistrate. “Rubber stamp” warrant processes where judges approve every application without meaningful review can be challenged.

Admin Per Se License Suspension – A.R.S. § 28-1385

Arizona’s Admin Per Se statute, A.R.S. § 28-1385, authorizes the MVD to suspend your driver’s license based solely on the BAC test results, independent of and prior to any criminal conviction. This administrative suspension is separate from the criminal case.

If your BAC test result is 0.08 or above, or if you refuse chemical testing, the arresting officer will serve you with an Admin Per Se / Implied Consent Affidavit. This document serves as notice that your license will be suspended:

BAC 0.08 or above: 90-day suspension (30 days no driving, then 60 days restricted with IID)
Test refusal: 12-month suspension (no restricted license for first 90 days)

You have only 15 days from the date of service to request a hearing to challenge the suspension. If you do not request a hearing within 15 days, the suspension takes effect automatically on the 16th day. This deadline is critical and cannot be extended.

The Admin Per Se hearing is an administrative proceeding before an administrative law judge, not a criminal court. The issues at the hearing are limited:

Did the officer have reasonable grounds to believe you were driving under the influence?
Were you placed under arrest?
Did you refuse the test, or did the test result show a BAC of 0.08 or above?
Were you informed of the consequences of refusal?

The hearing provides an opportunity to challenge the test results, the stop, and the arrest in an administrative forum. It also provides discovery opportunities that can benefit the criminal case. Testimony and evidence obtained at the Admin Per Se hearing can be used in the criminal defense.

Building an Effective Defense Against Chemical Test Evidence

Brad Rideout, as a former Arizona District Attorney, knows that the prosecution’s DUI case often rises or falls on the chemical test evidence. Challenge the test, and you challenge the core of the prosecution’s case.

Effective defense strategies include:

Independent Testing

Arizona law gives DUI defendants the right to obtain an independent test of their blood, breath, or urine at their own expense. If you requested an independent test and were denied, or if the officer failed to inform you of this right, the admissibility of the state’s test results may be affected.

Expert Witnesses

Toxicologists, forensic scientists, and laboratory quality assurance experts can testify about the limitations of BAC testing, the significance of procedural errors, and alternative explanations for test results. Expert testimony translates technical challenges into terms that jurors can understand and apply.

Discovery and Records Requests

Comprehensive discovery in a DUI case includes the instrument’s calibration records, maintenance logs, operator certification, quality assurance data, the laboratory’s proficiency testing results, the analyst’s training records, and the complete chain of custody documentation. Every link in this chain is a potential point of failure that the defense can exploit.

Frequently Asked Questions

Can I choose between a blood test and a breath test?

No. Under Arizona’s implied consent law, the officer selects the type of test. You do not have the right to choose. However, you do have the right to obtain an additional independent test at your own expense.

What if the officer did not observe me for 15 minutes before the breath test?

The 15-minute observation period is a standard protocol requirement designed to ensure that mouth alcohol does not contaminate the breath sample. If the officer did not comply with this requirement, the breath test results may be challenged as unreliable. Body camera footage is critical for establishing whether the observation period was properly conducted.

Can a blood test be wrong?

Yes. Blood tests, while generally more accurate than breath tests, are subject to errors from improper collection, contamination, fermentation, storage issues, and laboratory mistakes. No scientific test is infallible. The defense can challenge every step of the process from collection to analysis.

What is retrograde extrapolation, and can it be challenged?

Retrograde extrapolation is a mathematical calculation that estimates your BAC at an earlier time (time of driving) based on your BAC at a later time (time of testing). The calculation depends on assumptions about alcohol absorption and elimination rates that vary significantly between individuals. Defense experts can challenge the assumptions and demonstrate that the estimated BAC at the time of driving may have been lower than the prosecution claims.

Does refusing a blood test mean my case is dismissed?

No. Refusal triggers a 12-month administrative license suspension and allows the prosecution to argue consciousness of guilt at trial. Additionally, officers can and do obtain warrants for forced blood draws after a refusal. Refusal does not guarantee that the prosecution will lack BAC evidence.

What is the difference between a PBT and the Intoxilyzer 8000?

The Preliminary Breath Test (PBT) is a handheld device used at the roadside to help establish probable cause for arrest. PBT results are generally not admissible as evidence of your specific BAC at trial. The Intoxilyzer 8000 is the evidentiary breath testing instrument at the station, and its results are admissible. The two devices use different technology and have different accuracy levels.

Can medications affect my breath or blood test results?

Yes. Certain medications can affect breath test results by producing interfering compounds or affecting the body’s metabolism of alcohol. For blood tests, medications that affect liver function can alter alcohol metabolism rates. Inform your attorney about all medications you take so they can evaluate whether medication is relevant to your defense.

What if the blood draw was done at the hospital for medical purposes?

Hospital blood draws conducted for medical purposes use different procedures, equipment, and preservatives than forensic blood draws. Medical blood results may be less reliable for forensic purposes and can be challenged on that basis. However, prosecutors can and do use medical blood results as evidence in DUI cases, so this evidence must be addressed in the defense strategy.

A Former Prosecutor Who Knows the Evidence Inside and Out

Brad Rideout is a former Arizona District Attorney. He spent years relying on blood and breath test evidence to secure DUI convictions. He knows how the prosecution presents this evidence, how they handle challenges, and where the weaknesses in the testing process create opportunities for the defense.

That prosecution experience is the foundation of his defense strategy. When your attorney knows exactly what the prosecution’s case depends on, they know exactly where to attack it.

Contact Rideout Law Group Today

Scottsdale Office

11111 N Scottsdale Rd, Suite 225

Scottsdale, AZ 85254

Phone: (480) 584-3328

Lake Havasu City Office

2800 Sweetwater Ave A-104

Lake Havasu City, AZ 86406

Phone: (928) 854-8181

Toll-Free: (833) 854-8181

Call now for a confidential consultation. The blood or breath test in your case is not the final word. It is evidence – and evidence can be challenged. Brad Rideout will review the testing procedures, identify weaknesses, and fight to protect your rights.